1. Field
The disclosure relates to a method, a system, and a computer program product for the processing of splits of control areas and control intervals.
2. Background
Certain storage access methods may be used to access data stored in direct access storage devices. Data sets stored in direct access storage devices to which such storage access methods may be applicable may have certain characteristics. In such data sets, records may be placed into a data component based on the key value of the records. In certain situations, the records may be placed in ascending key value into the data component.
A control interval (CI) is a contiguous area of direct access storage that storage access methods use to store data records and control information that describes the data records. A control interval may be formed from one or more records or blocks. A control interval is comprised of records stored from beginning to end and may include free space for inserting or lengthening data records. Additionally, a control area (CA) is formed by two or more control intervals put together into fixed length contiguous areas of direct access storage, i.e., a plurality of control intervals are allocated in a single control area. A data set is stored in one or more control areas. In certain situations a control area may correspond to a cylinder having a plurality of tracks.
Control interval splits and control area splits occur as a result of data record insertions or as a result of an increase in the length of an already existing record. If a record is to be inserted (in key sequence) and there is not enough free space in the control interval, the control interval is split into two, i.e., a new free control interval is added to an originally existing control interval. Approximately half of the records in the originally existing control interval may be transferred to the new free control interval. The system then has two control intervals which are both about half empty, and the record to be inserted may be placed in the originally existing control interval. If more records are added to these control intervals and these control intervals fill up again, then the control interval splits may occur once again. It may be noted that as a result of a control interval split, the new control interval is allocated within the same control area in which the originally existing control interval is allocated.
If there are no free control intervals in an originally existing control area and a data record is to be inserted, a control area split occurs. As a result of the control area split, a single new control area is added to the originally existing control area for creating new space for the data record to be inserted. Therefore, the control area split results in two control areas being available to store data records instead of the single originally existing control area. Approximately half of the control intervals may be transferred to the single new control area and the other half may be stored in the originally existing control area. This movement creates space for new control intervals in the originally existing control area, and the record to be inserted may cause a control interval split for storing the record.